Television systems are known in which an interlaced video signal is converted to a non-interlaced or "progressively scanned" form in which the number of horizontal lines displayed in a field is doubled. Advantageously, such systems reduce the visibility of the line structure of displayed images.
Since doubling the number of displayed lines requires more lines than are actually transmitted, there have been a number of proposals for obtaining the required "additional" lines. An example of a system in which the required "extra" lines for display are obtained by repeating lines of a received signal is described by R. A. Dischert in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983. An example of a system in which the "extra" or interstitial lines are obtained by interpolation of adjacent vertical lines of the received signal is described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE-SCAN ARTIFACTS which issued Aug. 23, 1983. Other examples include the system described by Fujimura et al. in U.S. Pat. No. 4,509,071 entitled DOUBLE SCANNING NON-INTERLACE TELEVISION RECEIVER which issued Apr. 2, 1985 and the system described by Okada et al. in U.S. Pat. No. 4,451,848 entitled TELEVISION RECEIVER INCLUDING A CIRCUIT FOR DOUBLING LINE SCANNING FREQUENCY which issued May 29, 1984.
The above mentioned systems describe arrangements in which extra lines for display are derived from a currently received field of a video input signal. This form of progressive scan conversion is commonly known as "intra-field" or "line" conversion and has an advantage in that there are no visible artifacts produced for images containing field-to-field motion. However, there is a disadvantage in that the vertical resolution of displayed images is not improved and may be degraded, particularly where vertical interpolation is employed, and this tends to "soften" displayed images.
It has been widely recognized that the added lines needed for a progressive scan display can be obtained from a previous field rather than from a currently received field. Such systems are known generally as "field" or "inter-field" progressive scan systems and double the number of displayed lines by interleaving lines of a currently received field with lines of a previously received field. An advantage of "field" progressive scan processing is that still images are produced with the full vertical resolution of an originally scanned frame of video.
An example of a "field progressive scan" system is described by Okada et al. in U.S. Pat. No. 4,426,661 entitled TELEVISION RECEIVER INCLUDING A CIRCUIT FOR DOUBLING LINE SCANNING FREQUENCY which issued Jan. 17, 1984. See also, U.K Application GB 2,114,848A of Achiha et al. published Aug. 24, 1983 and entitled COLOR TELEVISION SIGNAL LINE DOUBLING CIRCUIT. Unfortunately, field progressive scan systems suffer from a problem in that if field-to-field motion exists in a scene the displayed images will be blurred. A further problem with progressive scan processors of the type in which extra lines are derived from a previous field is that a relatively substantial amount of memory is required for storing (delaying) the lines of the previous field.
It has been recognized, however, that one may obtain a desirable reduction in the memory requirements of a field type progressive scan system by utilizing only the low frequency components of the previous field and the high frequency components of a current field in forming the extra lines for display. Such a system is described in the Japanese laid open patent application of Tanaka et al., Kokai No. SHO 58-79379 entitled TELEVISION RECEIVER which was laid open on May 13, 1983. Although a desirable reduction in memory requirements is achieved in the Tanaka system, the problem of motion-related artifacts remains. An additional problem is that the system disclosed requires a pair of "matched" low-pass and high-pass filters for separating the video signal. Such filters require carefully selected amplitude and phase characteristics to separate high and low video signal components without forming a gap or overlap between them and are relatively complex and expensive.
Another example of a progressive scan processing system employing different processing of high and low frequency components is described by Dischert et al. in U.S. Pat. No. 4,673,978 entitled PROGRESSIVE SCAN PROCESSOR WITH PLURAL FREQUENCY BAND INTERPOLATION which issued Jun. 16, 1987. In this system the extra or "interstitial" lines for display are produced by adding a frame-combed and low pass filtered component of a video signal to a field delayed, line comb filtered and high pass filtered component of the video signal. The combined low and high frequency components are spatially and temporally coincident thereby reducing the visibility of motion artifacts (double images) during display. The system is "non-adaptive" in that the processing is not a function of scene motion.
Systems have been proposed in which the problem of vertical resolution characteristic of line-progressive scan systems and the problem of motion blur characteristic of field progressive scan systems has been approached by making the systems "motion adaptive". In motion adaptive systems a motion detector is used to switch between the two basic types of processors as a function of motion. For example, when the incoming video signal represents a still image the signal is processed by a field type processor which generates extra lines for display by interleaving the currently received lines with lines of a previous field. Conversely, when the incoming video signal represents a moving image, the extra lines for display are obtained by interpolation (or repeating) lines of the currently received field. For images that are not still and are not in full motion it is customary to "blend" or mix the outputs of line and field type processors in proportion to the magnitude of the motion.
Examples of "motion adaptive" progressive scan converters are described, for example, by Wargo et al. in U.S. Pat. No. 4,716,462 entitled MOTION ADAPTIVE TELEVISION SIGNAL PROCESSING SYSTEM which issued Dec. 29, 1987 and in U.S. Pat. No. 4,598,309 entitled TELEVISION RECEIVER THAT INCLUDES A FRAME STORE USING NON-INTERLACED SCANNING FORMAT WITH MOTION COMPENSATION which issued to R. F. Casey on Jul. 1, 1986.
A problem characteristic of motion adaptive systems is that the memory requirements for providing motion detection are far greater than the requirements for merely providing field progressive scan processing when motion detection is implemented in the usual way by comparing video signals delayed by one full frame. One approach to avoiding the requirement for a full frame of memory is to detect motion by measurement of the video signal sideband energy. An example of such a system is described by D. H. Pritchard in U.S. Pat. No. 4,641,186 entitled MOTION DETECTOR THAT EXTRACTS MOTION INFORMATION FROM SIDEBANDS OF A BASEBAND TELEVISION SIGNAL which issued February 1987. Although a full frame of memory is avoided, the sideband energy motion detection method is, however, relatively complicated.